Many compositions used for treating human skin impart unpleasant feel to the skin during or after use. Thus, many compositions containing soap, detergent or both and intended for cleansing the skin may dry the skin and leave it feeling rough, chapped and flaky. The drying effect of these compositions (which include toilet bars, liquid or powder hand washing and bubble bath compositions may be extremely pronounced in certain seasons, as in dry winter months. Use of such washing compositions during these seasons may dry the skin to the point of scaliness or brittleness, with resultant cracking, reddening, bleeding and soreness.
Other products applied to the skin such as shaving creams, foams or oils may also have a drying effect on skin. Moreover, the skin is stretched, scraped and often cut or punctured during shaving. These operations accompanying use of these products may exacerbate skin drying and increase skin feeling of roughness, flakiness and brittleness.
Products for the treatment of skin dryness include oils, balms, creams, lotions, liniments, ointments, unguents and gels. While these products may moisten skin and reduce or reverse roughness, cracking and brittleness of dry skin, many such products leave an unpleasant residue causing the skin to feel sticky, stiff, inflexible and waxy or unduly oily and greasy. Other products which may leave an unpleasant residue on the skin or which could benefit by improvement in skin treatment qualities include insect repellent and bite compositions, antiseptics and skin burn compositions (for burns from heat, sun or wind). Additionally, compositions for chapped lips such as lip balm or cream may impart an unpleasantly waxy or greasy feeling to the lips.
Citric acid derivatives have been incorporated in a wide variety of compositions. A salt of a monoester of citric acid, made from citric acid and alkanols of 1-18 or alkenols with up to 10 carbon atoms, is used to stabilize vinyl halide polymers in U.S. Pat. No. 3,362,923 (Kruth). Non-salt alkyl esters of citric acid in U.S. Pat. No. 3,929,712 (Hiyama et al.) are said to impart lubricant properties to vinyl chloride resins, each alkyl group having 12-22 carbons. Non-salt monoesters of citric acid are said to deactivate metals in petroleum products, e.g. cracked gasolines, according to U.S. Pat. No. 2,747,979 (Thompson). The radicals which esterify the citric acid may be alkyl or benzyl.
Monoesters of citric acid have also been used in treating clothes, as in U.S. Pat. No. 3,754,860 (Frick, Jr. et al.) where citrate monoesters with a small or moderate size aliphatic chain (up to 2-ethylhexyl) are included in wrinkle-resistant fabric finishes, and U.S. Pat. No. 3,971,626 (Heyden et al.) where a citric acid ester with a fatty alcohol of 12-22 carbon atoms is incorporated in a leather-treating agent. Particularly disclosed in Hayden et al. are citric acid esters with straight-chain unsaturated fatty alcohols and branched saturated or unsaturated fatty alcohols.
Monoesters of citric acid are also employed in the food technology art. U.S. Pat. No. 2,158,678 (Gooding et al.) describes agents said to retard the development of rancidity and improve moisture retention in glyceridic oil compositions, e.g., margarine. These agents, defined at col. 1, line 45 to col. 2, line 25, include monolauryl citrate and monostearyl citrate. Related U.S. Pat. No. 2,523,792 (Vahlteich et al.) describes edible compositions which are said to retard rancidity in glyceridic oils and which have 15 to 37.5% of selected monoesters of citric acid (including monolauryl, monomyristyl, monopalmityl, monooleyl and monostearyl citrate) dissolved in a solubilizing agent, e.g., lecithin. Monoesters of citric acid are also said to retard deterioration of milk and egg products in U.S. Pat. No. 2,667,419 (Gooding et al.). Citric acid monoesters of decanols, dodecanols, hexadecanols, and octadecanols are particularly disclosed and more particularly monolauryl and monostearyl citrate. Also, U.S. Pat. No. 2,902,372 (Harris) discloses monoesters of citric acid with aliphatic alcohols of less than 3 carbon atoms for the purpose of improving the whipping properties of egg whites. Finally, U.S. Pat. No. 3,004,853 (Julian et al.) discloses citric acid esterified with cetyl alcohol as part of an emulsifier system in a liquid shortening.
Other emulsifiers derived from citric acid, useful in the foods industry or cosmetology, are disclosed in U.S. Pat. No. 3,929,870 (David et al.). Specific citric acid derivatives in cosmetics include certain triesters of citric acid for shampoos disclosed in U.S. Pat. No. 4,176,176 (Cella) and 1-25% of citric or acetylcitric acid esterified by aliphatic alcohols having 1-6 carbon atoms for deodorant sticks and sprays disclosed in U.S. Pat. No. 4,010,253 (Reese et al.). European Patent 8105 (BASF) describes cosmetic preparations having 4-40% of citric acid esters carrying branched-chain alcohol radicals having 8-13 carbon atoms such as Tris-isodecyl citrate. German Patent No. 2,361,716 (Henkel) describes cosmetic preparations having 0.5-15% of coesters made from a) aliphatic diols, b) citric or acetylcitric acid, and c) aliphatic monofunctional alcohols having 12-30 carbon atoms. Preparations incorporating the coester are said to be soft and to produce no unpleasant feeling of sticking to the skin.
Citric acid or its derivatives have also been incorporated in toilet bars. Romanian Patent No. 72,330 (Grigorescu) describes incorporating citric acid in a cosmetic soap. U.S. Pat. No. 4,292,192 (Hooper) states that incorporating 0.3 to 3% of citric or acetylcitric acid esterified by an alkyl group of 1 to 4 carbon atoms imparts a deodorancy property to personal washing bars. (Another monoester incorporated in a personal washing composition, although not a citric acid derivative, is ROOC-(CH.sub.2).sub.n -COOM, where R is an alkyl or alkenyl chain with 4-12 carbon atoms, n=2-4 and M is a cation disclosed in the currently pending U.S. patent application of Nambudiry et al., Ser. No. 914,022, filed Oct. 1, 1986 for Detergent Compositions.)
Citric acid derivatives have also appeared in detergent compositions as pollution control substitutes for other components. Thus, U.S. Pat. No. 3,816,318 (Hentschel) discloses washing, dishwashing and cleaning detergent compositions which include 5 to 30% of salts of certain monoesters derived from polybasic carboxylic acids and alcohols of at least 1 hydroxyl group and 1-8 carbon atoms. Hentschel states that owing to strengthened lipophilicity, the monoesters have heightened emulsifying properties. Specific monoester salts are disclosed.
U.S. Pat. No. 4,271,032 (Kolaian et al.) discloses compositions for removing soil from fabrics in laundering. The compositions include monoesters of certain polycarboxylic acids, the alcohol radical having 12-30 carbon atoms. The monoesters are said to be effective as a surfactant or as a builder. European Patent No. 199,131 (Raffineria Olii Lubrificanti), published Oct. 29, 1986, describes surfactants derived from citric acid, namely citric acid mono-, di- and triesters with alkoxylated alcohols described therein. Mixtures of these esters are said to be very efficient surfactants with excellent detergent and biodegradability properties and with little or no toxicity or skin irritancy. Mixtures of the esters are also said to be suitable for a liquid detergent for kitchenware as well as liquid or creamy skin detergents or bath foam, as set out in the Examples.
It has been discovered that salts of certain monoesters of citric acid impart desirable qualities to skin treatment compositions.
Some conventional methods of synthesizing monoesters of citric acid yield a mix of mono-, di., and triesters of citric acid. The mixed mono-, di- and triester products of these methods is impractical in many applications. Only the monoester is soluble in alkaline aqueous systems. Additionally, the di- and triesters severely limit foaming. Thus, it would be desirable to produce monoesters of citric acid in a pure form, or in such predominance that the problems due to di- and triesters would be insignificant.
In the first of these methods, citric acid is reacted directly with an alcohol: the blend is heated, agitated until a solid forms, and cooled under reduced pressure, as described in U.S. Pat. Nos. 2,523,792 (Vahlteich) and 3,929,870 (David et al.).
A second method involves mixing citric acid in a dioxane solvent and adding an alcohol. The reaction mixture is refluxed for up to 72 hours, then the dioxane evaporated under reduced pressure. The remaining residue is diluted, then is stripped at 60.degree. C. under reduced pressure. This method is described in U.S. Pat. No. 4,271,032 (Kolaian et al.). The yield of monoester of citric acid from both these direct esterification methods is about 60%. It is noted that dioxane is not only an expensive solvent; its use may be accompanied by peroxide compounds which, upon accumulation, may be explosive.
While the methods of synthesizing monoesters of citric acid produce a mixture of mono-, di- and triesters, the relative molar amounts of citric acid and alcohol affect whether the mono, di- or triester product predominates. Thus, reacting substantially equal molar amounts of citric acid and alcohol (or a greater amount of the former) favors monoester, while diester predominates when a molar amount of alcohol double that of citric acid is used.
One process which exploits this effect is described in U.S. Pat. No. 2,518,678. Citric acid is dissolved in dry pyridine. Stearyl alcohol is added to the solution and heated for 20 hours. Since only a small concentration of stearyl alcohol is said to be soluble in the pyridine solution, a small concentration of the alcohol is continually reacted with a large concentration of citric acid, favoring monoester formation.
However, even this process produces a mixture of mono-, di- and triesters, requiring expensive and cumbersome steps to purify the monoester such as the techniques of fractional crystallization and selective extraction with suitable solvent systems described in U.S. Pat. Nos. 2,518,678 and 2,523,792.
A. J. Repta et al., "Synthesis, Isolation, and Some Chemistry of Citric Acid Anydride", Journal of Pharmaceutical Sciences 58, (September 1969), pages 1110-1114, describes synthesis of citric acid anhydride at page 1, col. 2, lines 14-31 and page 2. Suggested uses of the anhydride are as a dessicant or an ingredient in formulations for carbonation.
In the Nambudiry application cited above, monoesters are prepared by mixing and heating an acid anhydride, e.g. succinic anhydride, with fatty alcohol.
It has been discovered that monoesters of citric acid may be produced with minimal levels of di- and triester if citric acid is first reacted to form citric acid anhydride then reacted to form the monoester.